Fixation of a total hip replacement or other implant to the skeletal system is one of the most important problems in the field of orthopaedic biomaterials. Porous coated implants offer the potential for superior fixation and are being studied for use in a young, active patient population. However, porous coated implants are subjected to fatigue under the repetitive load of walking. It is known that porous coated metals have a reduced fatigue strength compared to the uncoated material. For example, porous coated Ti-6Al-4V, a typical .alpha.+.beta. titanium implant alloy, has a fatigue strength that is approximately one-third that of the uncoated material.
The reduction in fatigue strength has been attributed to two important factors. One factor is the microstructure that results from sintering the coating to the implant above the .beta. transus temperature. Experiments have been conducted to address this reason for the fatigue strength reduction of Ti-6Al-4V. As a result, the microstructure of porous coated Ti-6Al-4V has been modified utilizing post-sintering heat treatments that refine the lamellar structure most often found after the sintering procedure. A new, fine-grained structure which leads to a fatigue strength that is superior to that of the pre-sintered, equiaxed structure has been developed as a result. See U.S. Pat. No. 4,923,513--Ducheyne et al., which is incorporated herein by reference. However, although smooth specimen fatigue strength can be improved with changes in microstructure, porous coated specimen strength cannot. It has been found that porous coated Ti-6Al-4V has the same fatigue strength regardless of microstructure.
The second factor affecting fatigue strength is the interfacial geometry between the porous coating and the substrate, which creates stress concentrations. The effect of microstructure on fatigue strength reduction of porous coated Ti-6Al-4V is in fact outweighed by the effect of interfacial geometry with current porous coatings. Investigators have created numerical models of the interfacial geometry between the porous coating and substrate. See D. Wolfarth, et al., "The effect of stress concentrations on fatigue properties of porous coated implants," Trans. 13th Ann. Northeast Bioengineering Conference, Philadelphia, Pa. (Mar., 1987); P. B. Messersmith, et al., "Stress enhancement and fatigue susceptibility of porous coated Ti-6Al-4V implants: an elastic analysis," J. Biomed. Mater. Res., 24:591-604 (1990); D. Wolfarth, et al., "Parametric analysis of interfacial stress concentrations in porous coated implants," J. Applied Biomater., 1:3-12 (1990). These investigators have found that varying the interfacial radius affects the value of stress concentrations, quantified by the stress concentration factor, K.sub.t. It has also been found that varying the contact area between a coating particle and the substrate affects the value of K.sub.t. The measured values of interfacial radius and contact area on a porous coated hip stem has led to the discovery of a range of values for each which corresponded to a range in values of K.sub.t from 2 to 5.5. From these data it was concluded that by virtue of the current wide range in K.sub.t values, sintering process improvements to consistently produce a low value of K.sub.t around all sintered particles and hence an improved interfacial geometry for conventional coatings would not be reasonably possible. Thus, while it has been shown that an increase in sintering time and/or temperature may reduce the magnitude of the resulting stress concentrations, the interfacial radius, one important parameter that can be varied to reduce stress concentrations, is not significantly affected by specifics of the sintering process.
Therefore, there remains a need to improve the reduced fatigue strength of porous coated Ti-6Al-4V and other .alpha.+.beta. titanium alloys, as well as other alloys used in implants. The current structures present a serious impediment to the design of porous coated implants. For example, current porous coatings are often not applied to the lateral surface of hip stems because stresses are highest there. However, bone ingrowth occurs preferentially on the lateral surface. Furthermore, to make up for the reduced strength of the coated material, much larger prostheses are used. These prostheses have an increased cross-sectional moment of inertia and thereby minimize stress transfer to the surrounding bone. The net biological effect is that these prostheses lead to adverse bone tissue remodeling which can eventually lead to loosening of the device.
As explained above, current coatings cannot provide an interfacial geometry that leads to a consistent reduction in stress concentrations. Therefore, it would be desirable to determine the factors that create stress concentration factors in porous coated implants in order to provide a rational basis for analysis. Accordingly, it is an object of the present invention to minimize the stress concentration factor, K.sub.t, by selecting an optimal implant/coating geometry.